Takamasa Oshikiri

`Rect` mark of vega-lite was [introduced at vega-lite v2](https://github.com/vega/vega-lite/pull/1627), and Vegas currently [depends on vega-lite v1.2.0](https://github.com/vegas-viz/Vegas/blob/1496432875f80e9e579cc584fa8fd299f34a71a6/core/src/main/resources/webjars.csv ).

Hi, @hffmnn Honestly speaking, I am not actively maintaining and using this library. So I have no plan to implement these features But if someone submits a feature PR, I...